1. Technical Field of the Invention.
This invention broadly relates to concrete. It further relates to materials added to concrete or otherwise employed therewith to adjust the properties or to enhance and/or to facilitate the use thereof. This invention further, and more specifically, relates to an additive composition for and a method of promoting the flow of concrete and particularly the flow thereof through a pump and a conduit.
2. Description of the Prior Art and Problems Solved
It is well known that concrete is useful as a basic and highly versatile material of construction. The versatility of concrete stems, at least in part, from the fact that concrete initially occurs in a fluid condition, which, after a period of time, converts into a monolithic solid condition. In the fluid condition, concrete, being comprised of discrete solids dispersed in water, has little or no compressive strength; it can be pumped; and it will flow into and assume the shape of a container, referred to in the art as a form. After a period of time, the fluid concrete sets into the dense, rigid, monolithic solid condition having the shape of the form and exibiting substantial compressive strength.
For purposes of this disclosure, the fluid condition of concrete, which is referred to in the concrete construction art as "plastic concrete," featuring the dispersion of solids in water, is referred to herein as a concrete slurry, and the monolithic solid condition of concrete is referred to herein as set concrete. The dispersed solids are usually comprised of hydraulic cement and aggregate wherein the aggregate ordinarily, but not always, consists of graded coarse aggregate, i.e. rock, mixed with graded fine aggregate, i.e. sand.
It is known that a concrete slurry can be prepared for use in one location, which can be remote from the place of its intended use, and then conveniently transported by known means, such as in a truck, to the place of its intended use. It is also known that a concrete slurry can be prepared while being transported to the intended place of use. A truck ordinarily employed to transport a concrete slurry is referred to in the art as a ready mix truck.
When a concrete slurry is delivered to the intended location of use, it is known that it can be caused to flow by gravity or by pumping into a form where, because of its fluid condition, it conforms to the shape of the form. After the concrete slurry is in the form it is permitted to remain undisturbed therein for a period of time sufficient to enable the cementious material to hydrate to thereby produce the set concrete.
One of the problems involved in the placement of a concrete slurry, which is the problem specifically addressed in this disclosure, concerns the potential difficulty of causing the slurry to flow through a conduit. In this regard, it is sometimes necessary, or otherwise convenient, to place the slurry in the desired form by pressuring it, by means of a pump, through a conduit over a distance to the desired form. However, due to the nature of a concrete slurry, it has sometimes been found essential, in order to initiate movement of a slurry to and through the pump and conduit, to first fill the pump used to pressure the slurry through a conduit with a fluid which is easily pumped and which will lubricate the conduit itself, followed by pumping such fluid ahead of the slurry. The process of filling a pump with a fluid which is easily pumped followed by pumping in order to initiate slurry movement is referred to as priming.
It is known that a concrete slurry which contains coarse aggregate which is irregular in size and/or shape and/or which contains solids which are not uniformly dispersed in the water phase and/or which has a low volume ratio of hydraulic cement to total concrete slurry cannot be pumped easily, if at all, unless the pump is first primed.
A concrete slurry having a low volume ratio of hydraulic cement to slurry, which is known in the art as a low strength mix, does not contain sufficient hydraulic cement to lubricate the pump, conduit and aggregate to enable satisfactory slurry flow. It is believed that a low volume ratio of hydraulic cement to slurry is one featuring about 10 volume parts or less of hydraulic cement per 100 volume parts of concrete slurry.
The flow of a concrete slurry which includes coarse aggregate which is not of proper size gradation, as set out in ASTM C33-92a (Std. Spec. for Concrete Aggregates), or which is andgular or broken or in which solids are not uniformly dispersed in the water phase or any combination thereof will tend to be at least impeded and perhaps even will be completely blocked and will, accordingly, require the application of priming.
It is highly desireable that the combination of aggregate, water and hydraulic cement act together in the flowing slurry to produce a condition, referred to in the art as the ball bearing effect, which results in the smooth and uninterupted flow of the concrete slurry.
Coarse and fine aggregates often used in the formulation of a concrete slurry are not the type which will produce the ball bearing effect principally due to the fact that the more desireable type of coarse aggregates, i.e. aggregates which are round, smooth, unbroken and of proper size gradation, are in short supply or are simply not available in view of various factors including the high demand for concrete construction and decreasing convenient sources of such aggregates. Accordingly, the need for improved compositions for and methods of priming a concrete pump and lubricating a conduit to ease slurry flow is current and will continue to increase with the passage of time.
The prior art has addressed the problem of priming a concrete slurry pump by employing methods featuring the use of two different compositions, one of which is called a "priming grout mix" and the second of which is called a "priming slurry." The methods of priming a concrete slurry pump with either a priming grout mix or a priming slurry, as heretofore employed, are similar and suffer a common disadvantage. The disadvantage originates from the fact that the concrete slurry being pumped is not compatible and cannot be admixed with either one of the prior art priming compositions because any such admixture adversely affects the strength of the set concrete. Accordingly, when employing either one of the prior art methods, the delivery end of the conduit cannot be placed in the form until all of the priming composition has exited therefrom in order to avoid any admixing of slurry and priming composition.
Because the prior art compositions cannot be admixed with the concrete slurry, as explained above, the priming compositions must be directed to and placed in a separate location to enable hydration and subsequent disposal thereof. Accordingly, a waste disposal and potential environmental hazard inherently accompanies the use of prior art priming compositions.
Thus a problem to be solved is to devise a composition for and a method of priming a concrete slurry pump which will not only permit a slurry to be successfully pumped, but which will also avoid the waste disposal and environmental problems associated with methods currently employed.
A priming grout mix usually consists of concrete sand, hydraulic cement and water in the weight ratio of 10 to 2 to 1, respectively. In typical practice about one-half cubic yard of priming grout mix is placed in a ready mix truck which then transports the mix to the location of the concrete slurry pump, which is ordinarily a positive displacement pump; the mix is there employed to prime the pump in order to cause a concrete slurry to move through the pump and conduit to the desired form. In the example just cited, one-half cubic yard of priming grout mix includes about 1500 pounds of sand, 300 pounds of dry cement and 150 pounds of water. In view of the fact that a ready mix truck ordinarily holds at least about 8 cubic yards of material it is apparent that a truck employed to transport a priming grout mix is dramatically under utilized in that it is solely used to transport a material which cannot be added to the concrete slurry.
A priming slurry typically consists of one bag of hydraulic cement dispersed in 6 or 7 gallons of water. As used herein, a bag of cement weighs about 94 pounds; since a gallon of water weighs about 8.33 pounds, the weight ratio of cement to water in a typical priming slurry is an amount in the range of from about 1.6 to about 1.9 pounds of cement per pound of water. In practice about one-half of the required cement and water is thoroughly mixed and added to an apparatus, referred to in the art as a hopper, which is in direct fluid communication with the suction side of the concrete pump, thereafter the balance of the cement and water is thoroughly mixed and added to the hopper prior to the initiation of pumping.
Although use of a priming slurry does not involve an extra ready mix truck, as is required in the case of a priming grout mix as described above, it is required that extra bags of dry cement be transported to the pump location to enable on-site preparation of the the priming slurry.